The angular rate of rotation of a moving body about a rate axis is an essential input to a inertial navigation system. The angular rate of rotation about a given coordinate axis may be measured by vibrating an accelerometer along an axis normal to sensitive axis of the accelerometer and normal to the axis about which rotation is measured. From these data, the attitude of a vehicle or craft may be measured.
For example, an X, Y and Z coordinate system may be fixed in a moving body whose angular rate of change is to be measured. An accelerometer fixed in the body has its sensitive axis aligned along the Z axis. If the angular rotation vector of the body includes a component along the X axis, then periodic motion of the accelerometer along the Y axis will result in a periodic Coriolis acceleration acting in the Z direction. The magnitude of the Coriolis acceleration is proportional to the rotation rate about the X axis.
The accelerometer senses the Coriolis acceleration and generates an output signal. The output signal may include a slowly changing component which represents the linear acceleration of the body along the Z axis and a periodic component that represents the rotation of the body about the X axis. The accelerometer output can be processed, along with output signals of accelerometers having their sensitive axes in the X and Y directions and that are moved along the Z and X axes, respectively, to yield angular rate about the X, Y and Z axes, as described in U.S. Pat. No. 4,590,801.
Known types of accelerometers typically produce an output signal having a frequency related to the sensed acceleration. An example of such a frequency output accelerometer is a vibrating beam accelerometer. In this arrangement, a proof mass is supported by a flexure hinge or the like, and by a vibrating beam force sensing element that extends along the sensitive axis of the accelerometer. The force sensing element is coupled to a drive circuit that provides a signal for vibrating the force sensing element at its resonant frequency. An acceleration along the sensitive axis causes the proof mass to exert a tension or compression force on the force sensing element. A tension force on the force sensing element causes its resonant frequency to increase, while a compression force on the force sensing element causes its resonant frequency to decrease. The force sensing element can therefore be operated as a force to frequency converter in that frequency modulates an acceleration signal onto a carrier signal, the carrier signal being the zero acceleration resonant frequency of the force sensing element.
Other types of Coriolis rate sensors employ the use of a vibrating mechanism which is rotated in inertial space. Such an arrangement may be provided with a tuning fork having a pair of tines disposed in parallel relation. The tines are electromechanically excited in a plane at a predetermined frequency. The tines are interconnected by an output shaft, from which an output signal may be derived. The output signal is representative of the input angular rate of motion to which the body is subjected, which causes a deflection normal to the direction of vibration.
The output signal generated in these rate sensors may be converted to a digital signal, and may thereafter be conveniently processed in an inertial navigation system. The output signal is recovered by synchronous demodulation of signals produced by the Coriolis forces which are generated when a force sensing element is rotated in inertial space, as described, for example in U.S. Pat. No. 4,712,426.
Coriolis forces, however, are typically very small in many applications. Accordingly, very sensitive sensing devices with low noise characteristics must be utilized for accurate detection. Known solutions employ the use of piezoelectric materials. Such materials have been used, for example, in gravity wave detectors which are capable of detecting strains of 10.sup.-15. A practical difficulty, however, arises as the piezoelectric materials are fabricated in very small arrangements. A principal disadvantage of the materials used in known devices is that signal energy decreases dramatically with diminished size. The signal-to-noise ratio is therefore relatively poor.
Another disadvantage of known piezoelectric detection devices lies in the techniques used to fabricate such devices. Frequently, sensors are fabricated utilizing silicon micromachined structures. Piezoelectric materials, especially those with desirable sensing properties such as quartz or lead zirconate-titanate, are not easily combined with silicon technology. Accordingly, difficulties are frequently encountered in manufacture and implementation of sensitive detection devices.